An apparatus of the kind referred to above is disclosed in the international patent application WO 89/11082. This known apparatus comprises an inclined plane plate, on which the material passes slidingly, while the weight of the amount of material present on the plate at each instant is registered by the force it exerts against the plate. Thus, the weight of material being present on a known length of plate, in connection with a measured flow velocity, is utilized to determine the mass flow.
In addition to the requirement that this velocity measurement is to be carried out with the sufficient precision independently of the thickness of the layer of the flowing material (and the consequent differences in radius from the axis of the measuring drum), it is a prerequisite for this arrangement to function correctly, that the flow velocity on the whole plate is uniquely determined by the outlet velocity. This is normally not the case, as a coefficient of friction, that is smaller or greater than the tangent to the angle of inclination of the plate relative to the horizontal, will cause an acceleration or a deceleration respectively of the material, so that the mean velocity over the plate will be smaller or greater respectively than the outlet velocity as measured.